Genome characterization has enabled the cataloging of genes altered in human tumors and stimulated the development of therapies that exploit these alterations. Still, functional studies are ultimately needed to interpret and exploit the genetic variation that exists in human cancers. Furthermore, it is now apparent that cancer phenotypes and responses to therapy are dramatically influenced by the tissue microenvironment, and hence it is necessary to have in vivo models that accurately recapitulate both the genetics and physiology of cancers in patients. Although existing genetically engineered mouse models (GEMMs) have been instrumental in validating cancer-promoting mutations and developing therapeutic concepts in a physiological and relevant context, these models are simply too slow and expensive to be broadly useful and only recapitulate a minor fraction of the genetic lesions associated with human cancer. Driven by the need for more accurate and facile models, this project combines CRISPR genome engineering and in vivo organ electroporation with the goal of producing the first-in-kind collection of genetically-defined mouse models of three major epithelial malignancies. We refer to these models as electroporation-based genetically engineered mouse models (EPO-GEMMs). EPO-GEMMs have a range of advantages over traditional GEMMs in that they are fast, affordable, modular, highly portable, and avoid the substantial waste associated with GEMMs produced by strain intercrossing. These models are fully somatic, enable focal tumor development and, importantly, enable the study of tumor-host interactions by allowing tumors to be rapidly engineered in hosts of different genetic backgrounds. Based on substantial preliminary data to validate the EPO-GEMM concepts, our project will produce and characterize EPO-GEMMs of stomach, prostate, and pancreatic cancer - three common human cancers for which existing mouse models do not exist or are tedious. We will then perform a series of demonstration projects to evaluate and illustrate the unique potential of the EPO-GEMM approach, ranging from testing the efficacy and toxicity of target inhibition, exploring the effects of specific immune cell types on cancer initiation and progression, and using synchronous cohorts of genetically defined cancer models to test new targeted therapies and immune oncology approaches. Therefore, our project is of direct relevance to the overarching goals of the Oncology Models Forum, as EPO- GEMMs constitute ?translational research models that are robust representations of human biology, are appropriate to test questions of clinical importance, and provide reliable information for patients? benefit?. Each of these models will be credentialed with the Oncology Model Fidelity Score and all reagents will be made available through the NCIP Hub. We believe that the development and detailed characterization of rapid, flexible, and immunocompetent EPO-GEMMs and the adoption of these models for pre-clinical studies will be critical for the functional annotation of genetic variation in human cancer and greatly contribute to the implementation of precision oncology.